The deaf or hearing-impaired that cannot hear well enough to use the telephone use communication terminals to converse over telephone lines. Such terminals are referred to as telecommunications devices for the deaf or TDDs, and are comprised of a keyboard and display. TDDs are connected to a telephone line through a modem (modulator/demodulator) which allows the terminal to transmit information over a telephone line by coded tones to another eletronic device which is connected to the telephone line through another modem. TDDs were originally equipped to communicate via Baudot code.
As the telecommunications device, computer, and communications industries using ASCII (American Standard Code for Information Interchange) began to offer information and other services, it became desirable to have TDDs capable of operating in ASCII because communication using ASCII is faster, as well as there being other advantages in the use of ASCII. In those countries (such as the European Economic Community) that use CCITT instead of ASCII, it became similarly desirable to use CCITT instead of Baudot. Some TDDs now commercially available have the capability for both Baudot and ASCII (or, alternately, CCITT) available in the same machine. Present TDD multiple code devices include a hardware or software user selected switch to select which code is to be used in the communication. There are three codes now commonly used by TDDs: (1) Baudot at 45.5 baud, (2) Bell 103 ASCII Answer at 300 baud, and (3) Bell 103 ASCII Originate at 300 baud.
Problems often result because the TDD user does not always know whether the TDD, telecommunications device, or other device being used on the other end of the communication is capable of Baudot only, ASCII only, or both. Despite advantages inherent to the ASCII code, most TDD calls are made in Baudot because it is simpler to use and there are no settings that need to be made thus making it more attractive to lay users. To use ASCII code, both parties must agree on who will be in Originate mode and who will be in Answer mode. Optimization of the fastest (highest speed) code available to a particular set of users may also create difficulties. Due to the complexities of ASCII, persons who are not familiar with such codes and technologies are often confused and frustrated in attempting to deal with the many variables involved in determining the optimal code available for each communication and the necessary protocols.
An additional problem in the use of TDDs results when attempting to establish communication with a hearing person. The hearing person who answers the phone may not recognize the call as originating from a TDD because there is no information (codes or other sounds) present on the line when Baudot code is used and no information is currently being transmitted. The result can therefore sustain silence until the hearing party can recognize the situation and convert to the appropriate compatible code. Such a problem might be especially serious in an emergency situation. A similar communication problem may result if the answering party uses a TDD that is set to the noncorrespondent code.